RNA interference is currently recognized as a highly specific mechanism of sequence-specific gene silencing. See deFougerolles et al. (2007) Nature Reviews 6:443-453. The mechanism allows for the specific and profound reduction of proteins and mRNA.
Briefly, double-stranded RNA (dsRNA) is synthesized with a sequence complementary to a gene of interest and introduced into a cell or organism, where the dsRNA is recognized as exogenous genetic material and activates the RNAi pathway. If the exogenous dsRNA is relatively long, it will be cleaved into small interfering RNAs (siRNAs). Alternatively, if the exogenous dsRNA is relatively short (about 30 base pairs or less), cleavage does not occur, the exogenous dsRNA itself acts as the siRNA substrate, and complications arising from activation of innate immunity defenses are avoided. In both cases, the siRNA becomes incorporated into an RNA-induced silencing complex (RISC) followed by unwinding of the double stranded siRNA into two strands. One of these strands, the “sense” strand (also known as the “passenger” strand), is discarded. The other strand, the “guide” strand (also known as the “antisense” strand) recognizes target sites to direct mRNA cleavage, thereby silencing its message. A similar RNAi mechanism involves microRNAs (miRNAs) deriving from imperfectly paired non-coding hairpin RNA structures.
Through the specific targeting of genes, RNAi-based therapies have the ability to substantially block the production of undesired proteins. Thus, in diseases and conditions attributable to the undesired or over expression of certain proteins, RNAi-based therapies represent a potentially powerful and important approach.
Despite the great promise of RNAi-based therapies, there remains a problem of the relative short half life of these therapeutics in vivo. There remains a need for better and improved versions of siNA in order to bring the RNAi-based therapies to fruition.